The gastrointestinal (GI) system has its own nervous system, the enteric nervous system (ENS). Neurogastroenterology is the study of the enteric nervous system, a subdivision of the autonomic nervous system (ANS) that directly controls the gastrointestinal system. The ENS is capable of autonomous functions such as the coordination of reflexes. Although it receives considerable innervation from the autonomic nervous system, it can and does operate independently of the brain and the spinal cord. The ENS consists of some 100 million neurons, one-thousandth of the number of neurons in the brain, and about one-tenth the number of neurons in the spinal cord. The enteric nervous system is embedded in the lining of the gastrointestinal system.
GANGLIA OF THE ENS
The neurons of the ENS are collected into two types of ganglia: myenteric (Auerbach's) and submucosal (Meissner's) plexuses. Myenteric plexuses are located between the inner and outer layers of the muscularis externa, while submucosal plexuses are located in the submucosa.
The myenteric plexus is mainly organized as longitudinal chains of neurons. When stimulated, this plexus increases the tone of the gut as well as the velocity and intensity of contractions. This plexus is concerned with motility throughout the whole gut. Inhibition of the myenteric system helps to relax sphincters - the muscular rings that control the flow of digested food or food waste.
The submucosal plexus is more involved with local conditions and controls local secretion and absorption, as well as local muscle movements. The mucosa and epithelial tissue associated with the submucosal plexus also have sensory nerve endings that feed signals to both layers of the enteric plexus. These tissues also send information back to the sympathetic pre-vertebral ganglia, the spinal cord, and the brain stem .
Neural control of the gut
An illustration of neural control of the gut wall by the autonomic nervous system and the enteric nervous system.
FUNCTION AND STRUCTURE
The enteric nervous system has been described as a "second brain. " There are several reasons for this. The enteric nervous system can operate autonomously. It normally communicates with the central nervous system (CNS) through the parasympathetic (e.g., via the vagus nerve) and sympathetic (e.g., via the prevertebral ganglia) nervous systems. However, vertebrate studies show that when the vagus nerve is severed, the enteric nervous system continues to function.
In vertebrates, the enteric nervous system includes efferent neurons, afferent neurons, and interneurons, all of which make the enteric nervous system capable of carrying reflexes and acting as an integrating center in the absence of CNS input. The sensory neurons report on mechanical and chemical conditions. Through intestinal muscles, the motor neurons control peristalsis and churning of intestinal contents. Other neurons control the secretion of enzymes. The enteric nervous system also makes use of more than 30 neurotransmitters, most of which are identical to the ones found in CNS, such as acetylcholine, dopamine, and serotonin. More than 90% of the body's serotonin lies in the gut, as well as about 50% of the body's dopamine, which is currently being studied to further our understanding of its utility in the brain.
The enteric nervous system has the capacity to alter its response depending on such factors as bulk and nutrient composition. In addition, ENS contains support cells, which are similar to astroglia of the brain, and a diffusion barrier around the capillaries surrounding ganglia, which is similar to the blood-brain barrier of cerebral blood vessels.
REGULATION OF ENS FUNCTION
The parasympathetic nervous system is able to stimulate the enteric nerves in order to increase enteric function. The parasympathetic enteric neurons function in defecation, providing a rich nerve supply to the sigmoid colon, the rectum, and the anus.
Conversely, stimulation of enteric nerves by the sympathetic nervous system will inhibit enteric function and capabilities. Neurotransmitter secretion and direct inhibition of the enteric plexuses cause this stall in function. If the gut tract is irritated or distended, afferent nerves will send signals to the medulla of the brain for further processing.